Antireflective films have been employed to prevent the reflection of background and improve the visibility in image display units such as a cathode-ray tube (CTR), a plasma display panel (PDP), an electro luminescence display (ELD) and a liquid crystal display (LCD).
An antireflective film is located as the outermost face of a display so as to lower the refractive index with the use of the optical interference principle, thereby preventing a decrease in contrast or image transfer caused by the reflection.
Thus, an antireflective film is highly likely injured and it is therefore an important problem to impart an excellent scratch resistance to it.
Such an antireflective film can be constructed by providing a low-refractive index layer having an appropriate membrane thickness as the outermost face and optionally forming an high-refractive index layer, a medium-refractive index layer, a hard coat layer and so on between the low-refractive index layer and a support (a substrate). In order to achieve a low refractive index, it is preferred to employ a material having a refractive index as low as possible as the low-refractive index layer. Since the antireflective film is employed as the outermost face of a display, it should have a high scratch resistance. To achieve a high scratch resistance in a thin film having a thickness of about 100 nm, it is required that the film per se has a strength at a certain extent and a high adhesiveness to the under layer.
The refractive index of a material can be lowered by introducing a fluorine atom into a monomer binder having an ethylenically unsaturated group, or by lowering the density (introducing voids). By using these methods, however, the film strength and adhesiveness are worsened and the scratch resistance is lowered. Namely, it is a difficult problem to establish both of a low refractive index and a high scratch resistance.
JP-A-11-189621 proposes a method comprising introducing a polysiloxane structure into a fluoropolymer to thereby lower the coefficient of friction of the film surface and improve the scratch resistance. Although this method is efficacious to a certain degree in improving the scratch resistance, it is impossible to impart a sufficient scratch resistance by this method alone to a film which inherently has an insufficient film strength and interfacial adhesiveness.
On the other hand, it is stated in JP-A-2002-156508 that the hardness of a photo curing resin is elevated by curing it at a low oxygen concentration. To produce an antireflective film by using a web at a high efficiency, however, the degree of lowering the oxygen concentration by, for example, nitrogen purge is limited and thus no sufficient hardness can be obtained thereby.
JP-B-7-51641 discloses a method of twisting a base material around a hot roll and irradiating with ionizing radiation. However, the degree of curing by this method is still insufficient in the case where it is intended to sufficiently cure a thin film for specific use such as a low-refractive index layer.